1. Field of the Invention
The present invention relates to a single-lens reflex type camera having a solid-state image sensor and arranged to be capable of recording information by using the solid-state image sensor and to permit use of an interchangeable photo-taking lens.
2. Description of Related Art
A number of cameras of the single-lens reflex type having a light flux obtained through a photo-taking lens used both by a viewfinder optical system for observing a photo-taking object and by an image forming system for taking a picture of the object have been developed to use a solid-state image sensor in place of a silver-halide film at an image recording part. However, if accessories used for the conventional single-lens reflex cameras, such as interchangeable lenses, external electronic flash devices, etc., are not usable for the cameras having the solid-state image sensors, it is greatly disadvantageous not only to the users of conventional single-lens reflex cameras but also to the providers of the cameras.
Accordingly, a number of cameras have been variously proposed in which, to provide a camera having a solid-state image sensor without varying the system interchangeability of single-lens reflex cameras, the solid-state image sensor is placed at a focal image forming portion corresponding to the position of the silver-halide film on the base of the conventional silver-halide-film-using single-lens reflex camera.
In the case of such a camera having a solid-state image sensor, a CCD image sensor or the like is used in general. However, such an image sensor has a spectral sensitivity characteristic which requires some correction. To meet this requirement, some optical filters are used in general. These filters include a low-pass filter which is used for avoiding a moire pattern resulting from a relation between the number of pixels of the image sensor and the high spatial frequency of the photo-taking object and for avoiding a spurious color obtained in the case of a color image sensor, and an infrared cut filter for cutting off an infrared component. As disclosed in Japanese Patent Publication No. HEI 4-435, the optical filter can be disposed in a space immediately before a shutter.
However, such a disposition of the optical filter in that space brings about the following two problems.
Firstly, unlike the conventional single-lens reflex camera, the camera of the kind having a solid-state image sensor requires no film replacing action and thus requires no back lid (back cover) arranged to be opened and closed for replacing a film. Therefore, if it is taken into consideration that the work of removing dust attached to the optical filter or replacing the optical filter with another optical filter is performed through a lens-mount opening part of the camera, the existence of a main mirror makes it very difficult to perform that working.
Secondly, since a light flux coming from a photo-taking object is split by the main mirror into light fluxes which are directed respectively toward a viewfinder system and a photo-taking system and, then, only the light flux directed to the photo-taking system passes through the optical filter, a change in optical path length caused by the optical filter takes place only in the photo-taking system. Hence, the optical path length of the viewfinder system must be made to be equal to that of the photo-taking system either by providing the viewfinder system also with an optical filter or by modifying the design of optical path length of the viewfinder or that of the total optical path length of the photo-taking system. However, such a modification not only necessitates redesign work but also results in an increase in size of the camera.
To solve the above-stated two problems, it has been considered to change the installing position of the optical filter to a position immediately before the main mirror, i.e., immediately behind the photo-taking lens.
Such a change in the installing position of the optical filter solves the above first problem to facilitate the above-stated optical filter cleaning and replacing work, because the optical filter is located immediately behind the lens-mount opening part.
As for the above second problem, since the object light flux is split by the main mirror to the viewfinder system and the photo-taking system after passing through the optical filter, the change in optical path length is equally caused by the optical filter both in the viewfinder system and the photo-taking system. Therefore, a change in the focal image forming position resulting from an increase of the optical path length due to the optical filter can be corrected by just changing the thickness of the mount.
FIG. 7 shows by way of example the arrangement of the conventional single-lens reflex type camera having a solid-state image sensor in which an optical filter is installed immediately before a main mirror. Referring to FIG. 7, a bayonet mount part 2 is provided at the front surface of a camera body 1. A photo-taking lens (not shown) is detachably mounted on the mount part 2. An optical filter 3 which is a low-pass filter or an infrared cut filter is disposed between the mount part 2 and the main mirror 4. When the user of the camera observes an object image, an object light flux coming through the photo-taking lens passes through the optical filter, so that a spectral sensitivity characteristic of the object light flux is corrected there. After the correction, a part of the object light flux is reflected upward almost at a right angle by the main mirror 4, which is a half-mirror. The reflected object light flux passes through a focusing screen 5 and a condenser lens 6 to be converted into an erecting image by a vertical and lateral inverting action of an erecting image forming element 7. The light flux of the erecting image thus obtained is led to the user through an eyepiece lens group 8. In addition, another part of the object light flux passes through the main mirror 4 and is then totally reflected by a total-reflection submirror 9. The light flux reflected by the submirror 9 is led to an automatic focusing sensor 13 through an automatic focusing optical system (10, 11 and 12) so as to detect a focusing state of the photo-taking lens.
The optical filter 3 is interposed in between a support member 14 and a fixing plate 15 to be fixed in position perpendicularly to an optical axis and not to become aslant. The support member 14 is mounted, integrally with the optical filter 3, on the camera body 1 with a screw (not shown).
In taking a shot at an object, the main mirror 4 and the submirror 9 required for the above-stated observation and focus detection are beforehand retracted from the path of the object light flux between an objective lens group of the photo-taking lens and a CCD image sensor 16 serving as a focal plane.
Next, a photo-taking light flux having passed through the objective lens group of the photo-taking lens passes through the optical filter 3 to have a spectral sensitivity characteristic thereof corrected there. When a fully-open state of shutter leading blades (front curtain) 17a is obtained, an image of the photo-taking object is formed on the CCD image sensor 16, which is mounted from the side of a back lid 18 in a focal image forming position that is about the same as the position of a silver-halide film in a single-lens reflex camera. After the CCD image sensor is exposed to the light of the object image to accumulate it for a predetermined period of time, the light flux of the photo-taking object image is cut off by a closing action of shutter trailing blades (rear curtain) 17b. After that, information on the image thus picked up is transferred to be recorded as an image signal on a storage device disposed either within the camera body or outside of the camera body.
The single-lens reflex type camera arranged to have the optical filter mounted immediately before the main mirror as described above, however, has the following problem. Since the main mirror 4 is arranged to be retracted upward at the time of an exposure, the size of a flange back part must be increased for the purpose of preventing the main mirror 4 from interfering with the optical filter 3. This increase causes an increase in size of the camera.
This problem may be solved by arranging the main mirror 4 in a smaller size to prevent the main mirror 4 from interfering with the optical filter 3.
However, in mounting a photo-taking lens on the above-stated camera having a solid-state image sensor, the base of which is the conventional single-lens reflex camera using a silver-halide film, an end part of the photo-taking lens tends to damage, or might break, the surface of the optical filter if the end part of the photo-taking lens comes to touch the optical filter. This problem still remains to be solved.